Lithium ion secondary batteries including an alloy-formable active material as a negative electrode active material (hereinafter referred to as “alloy-type secondary batteries”) have higher capacities and higher energy densities than conventional lithium ion secondary batteries including graphite as a negative electrode active material. Therefore, alloy-type secondary batteries are expected not only as power sources for electronic equipment, but also as main power sources or auxiliary power sources for transportation equipment or machining equipment. Examples of a known alloy-formable active material include a silicon-based active material such as silicon and a silicon oxide, and a tin-based active material such as tin and a tin oxide.
In an alloy-formable active material, internal stress is generated during charge due to excessive expansion of its particles. This may cause separation of the negative electrode active material layer from the negative electrode current collector, or deformation of the negative electrode, and thus, the cycle characteristics of the battery may deteriorate. In order to solve such a problem of an alloy-formable active material, it is proposed to form a coating made of a resin material on an outer surface of the negative electrode active material layer including an alloy-formable active material.
Patent Literature 1 discloses a negative electrode in which a polymer film layer composed of a polymer support and a cross-linkable monomer is formed on an outer surface of a negative electrode active material layer including lithium alloy particles.
Patent Literature 2 discloses a negative electrode including: a negative electrode current collector; a negative electrode active material layer being supported on a surface of the negative electrode current collector, including an alloy-formable active material, and having recesses and protrusions on its outer surface; and a resin layer formed in the recesses, wherein the outer surface of the resin layer is flushed with the outer surfaces of the protrusions.